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Abstract:

An optical receiver lens has a three-dimensional lens surface, for
receiving the laser radiation of a laser distance measuring device, said
laser radiation being reflected at an object, wherein the receiver lens
can be described in a three-dimensional coordinate system having three
axes x, y, z arranged at right angles with respect to one another and
wherein the z-axis coincides with the optical axis of the receiver lens.
At least one non-spherical area section of the lens surface can be
described by addition of a first area, the flexure of which along the
z-axis is a first function (f1) of x and y, in particular of (I) and
a second area, the flexure of which along the z-axis is a second function
(f2) of x and not of y. A distance measuring device is also
described.

Claims:

1. An optical receiver lens having a three-dimensional lens surface, for
receiving the laser radiation, reflected at an object, of a laser
distance measuring device, the receiver lens being defined in a
three-dimensional coordinate system having three axes x, y, z arranged at
right angles to one another, and the z-axis coinciding with the optical
axis of the receiver lens, wherein at least one aspheric surface section
of the lens surface is defined by the addition of a first surface, whose
sag along the z-axis is a first function f1 of x and y, in
particular of {square root over (x2+y2)}, and of a second
surface, whose sag along the z-axis is a second function f2 of x and
not of y.

2. The optical receiver lens as claimed in claim 1, wherein the first
and/or the second function are/is at least once continuously
differentiable.

3. The optical receiver lens as claimed in claim 1, wherein the aspheric
surface section of the lens surface, or a surface subsection of the
aspheric surface section is defined by the supplementary addition of a
third surface, whose sag along the z-axis is a third function, in
particular of x and/or y.

4. The optical receiver lens as claimed in one claim 1, wherein the
aspheric surface section is arranged on a receiver lens side facing the
reflecting object.

5. An optical distance measuring device, having an optical receiver lens
having a three-dimensional lens surface, for receiving the laser
radiation, reflected at an object, the receiver lens being defined in a
three-dimensional coordinate system having three axes x, y, z arranged at
right angles to one another, and the z-axis coinciding with the optical
axis of the receiver lens, wherein at least one aspheric surface section
of the lens surface is defined by the addition of a first surface, whose
sag along the z-axis is a first function f1 of x and y, in
particular of {square root over (x2+y2)}, and of a second
surface, whose sag along the z-axis is a second function f2 of x and
not of y.

6. The optical distance measuring device as claimed in claim 5, wherein
the optical receiver lens is the single receiver lens and/or the single
optical element in the receiving beam path.

7. The optical distance measuring device as claimed in claim 5, wherein
the receiving beam path is arranged at a parallax angle to a transmit
beam path.

8. The optical distance measuring device as claimed in claim 7, wherein
the optical receiver lens is arranged in such a way that the transmit
beam path is intersected by the x-axis.

Description:

PRIOR ART

[0001] The invention relates to an optical receiver lens having a
three-dimensional lens surface in accordance with the preamble of claim
1, and to an optical distance measuring device, in particular a laser
distance measuring device, in accordance with claim 5.

[0003] Difficulties in the design of receiver optics for distance
measuring devices stem from the stipulation that the distance measuring
devices be capable of use for precise measurement both in the near range
and in the far range. Problems in the measurement of large distances
arise from extraneous light influences that have a negative effect on the
signal-to-noise ratio. In order to reduce the extraneous light influence,
the size of the photodetector that is used is usually tuned as well as
possible to the size of the light point reflected by distant objects.
With near field measurements, there is, inter alia, the problem that a
parallax angle between an emitted light beam and a received light beam
has a comparably strong effect on the measurement result. The parallax
angle is to be ascribed to the fact that the transmitting lens system is
arranged next to the receiver lens system. A further, important problem
in the design of distance measuring devices that are intended to be used
both for the near range and for the far range consists in that the
received optical power is proportional to the inverse distance squared in
the case of large distances. This results in the necessity to design the
receiving system for processing weak signals. The sharp increase in the
received laser power in the case of short distances lead, however, at the
same time to the fact that the receiving system must be designed to be
comparably inefficient for short distances in order to prevent saturation
of the electronic detection circuitry.

DISCLOSURE OF THE INVENTION

[0004] It is the object of the invention to propose a receiver lens that
is suitable for use in measuring both in the near range and in the far
range. Furthermore, the object consists in providing an optical distance
measuring device with a correspondingly improved optical receiver lens.

[0005] This object is achieved by the features of claim 1 with regard to
the optical receiver lens, and by the features of claim 5 with regard to
the optical distance measuring device. Advantageous developments of the
invention are specified in the subclaims. All combinations of at least
two of the features disclosed in the description, the claims and/or the
figures fall within the scope of the invention.

[0006] The invention is based on the idea of designing at least one
aspheric surface section of the lens surface, preferably an aspheric
surface section of a lens surface facing the reflecting object, in such a
way that it is obtained by an addition of at least two, preferably of
exclusively two, surfaces, specifically of a first surface whose sag
along the z-axis, that is to say whose extent along the z-axis, is a
function both of x and y, in particular the radius
(r2=x2+y2), and of a second surface whose sag along the
z-axis is exclusively a function of x, that is to say not of y. It is
particularly preferred for the aspheric surface section to be designed in
this case in such a way that the ratio between the optical power received
by the optical receiver lens and the optical power detected by a
photodetector rises with increasing distance until a constant ratio of
preferably more than 90% is reached. The result of this is that the
optical power on the photodetector, that is to say the detector
circuitry, is not saturated even in the near range and decreases toward
smaller distances. A distance measuring device equipped with an optical
receiver lens designed using the concept of the invention is
distinguished by an optimized signal-to-noise ratio. The receiver lens is
further distinguished by the possibility of being used for measurements
in the near range and in the far range. In particular, a good, stable
signal amplitude can be obtained without further outlay even for short
distance measurements. Moreover, it is possible for the receiver lens to
be formed cost-effectively of plastic. Furthermore, a preferably
implemented parallax angle between a receiver beam path and a transmit
beam path has only an unimportant effect on the measurement result.

[0007] In a development of the invention, it is advantageously provided
that the first function describing the sag of the first surface along the
z-axis, and/or the second function describing the sag of the second
surface along the z-axis are/is at least once continuously
differentiable. The lens surface can be produced more simply as a result.
It is particularly advantageous when the functions describing the sag are
at least twice continuously differentiable.

[0008] It is advantageously provided in the development of the invention
that in addition to the first and the second functions for describing the
aspheric surface section or a surface subsection of the aspheric surface
section it is possible to add at least a third surface whose sag along
the z-axis can be described by a third function, for example by a
function dependent on x and y, such as, for example:

sag ( x , y ) = f ( x ) y 2 1 + 1 - f ( x )
2 y 2 ##EQU00001##

[0009] It is very particularly preferred when the aspheric surface
section, formed as previously described, of the lens surface is arranged
on a receiver lens side facing the reflecting object.

[0010] The invention also leads to an optical distance measuring device,
in particular a laser distance measuring device, having an optical
receiver lens formed as previously described. The optical distance
measuring device can be designed to operate in a way known per se on the
basis of interferometric measurements, and/or on the basis of flight time
measurements.

[0011] Preference is given to an embodiment of the distance measuring
device in the case of which the optical receiver lens is the sole
receiver lens in the receiving beam path. It is very particularly
preferred not to provide any further optical elements, such as mirrors,
etc., in addition to the sole receiver lens.

[0012] Particular preference is given to an embodiment of the distance
measuring device in the case of which the receiving beam path is arranged
at a parallax angle to a transmit beam path, the transmit beam path
being, with very particular preference, intersected at right angles by
the x-axis of the three-dimensional coordinate system describing the
receiving lens.

[0013] Further advantages, features and details of the invention emerge
from the following description of preferred exemplary embodiments, and
with the aid of the drawings, in which:

[0016]FIG. 2b show two views, rotated by 90°, of an optical
receiver lens, the position of a three-dimensional coordinate system with
the axes x, y and z being clear from the views,

[0017]FIG. 3 shows a possible arrangement of an optical receiver lens in
a distance measuring device,

[0018]FIG. 4a shows a diagram in which the ratio of the optical power
received by the optical receiver lens to the optical power detected by a
photodetector is plotted against the distance to be measured,

[0019]FIG. 4b shows a further diagram, which shows the optical power
striking the photodetector plotted against the distance to be measured,

[0020]FIG. 5 shows an optical lens having an aspheric surface section,
and

[0021]FIG. 6 shows a description of a second function, dependent
exclusively on x, by the piecewise linked second order polynomials.

[0022]FIG. 1 shows an optical receiver lens 1 for a distance measuring
device 2 illustrated in FIG. 2 and designed as a laser distance measuring
device. A three-dimensional lens surface 3 facing the reflecting object
(not shown), whose distance can be determined by means of the distance
measuring device 2 comprises a spherical section 4 on the left in the
plane of the drawing, an adjacent aspheric surface section 5, to the
right thereof in the planes of the drawing, for medium distances, and an
aspheric surface section 6, adjacent in turn to the latter surface
section 5, for short distances. The surface sections marked with the
reference numeral 7 do not have a receiving function.

[0023] FIGS. 2a and 2b illustrate the position of a receiver lens 1 in a
three-dimensional coordinate system comprising the three axes (x, y, z)
running at right angles to one another. It is to be assumed that the
x-axis and the y-axis characterize the surface extent of the lens surface
3, whereas the z-axis, which is at right angles to the surface extent of
the lens surface, coincides with the optical axis of the receiver lens 1.

[0024] A possible arrangement of the receiver lens 1, which is preferably
formed from plastic, in a distance measuring device 2 emerges from FIG.
3. It is to be seen that the receiver lens 1 is arranged on the left next
to a lens 8 of a transmit beam path, and this leads to a parallax angle
(not illustrated) known per se between the transmit beam path and a
receiving beam path radiating through the receiver lens 1.

[0025]FIG. 4a shows a diagram in which a ratio value v, formed from an
optical power received by the receiver lens 1, and an optical power
received by a photodetector, is plotted logarhythmically on the ordinate,
and in which the distance is plotted logarhythmically on the abscissa. It
is to be seen that the ratio value V in the logarhythmic representation
rises linearly with the gradient 2, ideally as far as 100% (in practice,
less than 100%, but greater than 90%) and then remains constant. As is
to be gathered from the diagram in accordance with FIG. 4b, the result of
this is that the optical power P of the photodetector is constant at
first, that is to say in the near range, and decreases starting from a
specific distance value. The diagram in accordance with FIG. 4b also uses
logarhythmic coordinate axes.

[0026] It is to be noted in general that the sag along the z-axis in the
case of a receiver lens designed according to the concept of the
invention can be determined by the addition of a first function
f1(x+y) and of a second function f2(x). That is to say, as a
sum of a first function which is dependent on {square root over
(x2+y2 )} and a second function which is dependent exclusively
on x and not on y.

[0027] An example of a first and a second function is explained below by
means of FIGS. 5 and 6. The sag of the aspheric surface section, shown in
FIG. 5 with the reference numeral 9, along the z-axis is described by
adding these functions f1, f2. A spherical surface section 4
for the far range is located on the left next to the aspheric surface
section 9 in the plane of the drawing.

[0028] In accordance with a preferred embodiment, the first function is